Mark Cerny on PS4: GDDR5 Latency Not "Much of a Factor"

There have been voices of concern around the internet about the 8 Gigabyte of GDRR5 of the PS4 suffering from latency compared to to DDR3, but the console's Lead Architect Mark Cerny isn't particularly worried about that.

I think he was just saying that if PS3 which i known to have memory problem, and somehow GTA5 was still made on that system, then the PS4 will be fine as some clever developers will find a way around any small latency issues that may or may not be a problem, and still make amazing games...

Now to go more in depth- I think we have to understand what Latency is first- and how it is being talked about here in the PS4

Latency -latency is the time (either in clocks or nano-seconds) taken to transfer a block of data either from main memory or GPU caches. We want the data as quickly as possible, thus the lower the time the better. The size of the data block we request is usually the size of a native pointer (4 bytes in 32-bit, 8 in 64-bit).

As a GPU (or APU) executes instructions, both the instructions themselves and the data they operate on must be brought into registers; until the instruction/data is available, the GPU cannot proceed and must wait; even advanced designs that can execute out-of-order eventually need data.

Latency is generally measured in core "clocks" (1/frequency) for caches (as they usually run at GPU speed) and nano-seconds (10^-9) for the main memory.

- TO MAKE IT SIMPLE - The latency of the main memory directly influences the efficiency of the GPU and from what I seen of the PS4, this will not be an issue in the PS4.

Memory latency was never an issue with the GPU, as GDDR5 is what graphics card use by standard pretty much. So it doesn't get worse than it.

The key here is that it affects CPU and you might get frequent CPU cycles stall. Cerny cleverly deflects that, by only talking about the GPU and most journalist don't have the tech knowledge (for good reason) to ask the proper question.

In fact on PC, the graphics memory has to fetch the data from main memory first, so that indicates latency isn't a issue. However, on PC the CPU deals with DDR3 which has significantly lower latency than GDDR5 thus, but on PS4 that latency on GDDR5 carries over to GPU.

Cerny himself said:

"Latency in GDDR5 isn’t particularly higher than the latency in DDR3. On the GPU side… Of course, GPUs are designed to be extraordinarily latency tolerant so I can’t imagine that being much of a factor."

The reason latency is not an issue for the CPU is because the CPU is clocked at half the frequency of a normal CPU, 1.6 ghz instead of 3.2 ghz. If the CPU was faster than I could foresee latency issues. There is a reason the CPU is clocked at 1.6 ghz and not 2.4 or 3.2 ghz.

Latency isn't the time taken to transfer a block of data, it's the time it takes to START transferring the data.

For the GPU this tends to not be much of an issue, it makes far fewer requests for data but those requests tend to be for large chunks of data so the time it takes to start transferring data isn't as important as how fast that data can actually be moved. That's why you use GDDR5 memory for graphics, it's slow to start moving the data but it can move a lot of data quickly. Yes the GPU has to wait for the data to start coming but after that it isn't starved for data.

For the CPU it is the opposite, it makes far more requests for small pieces of data. It's more important that the data starts transferring sooner than how much data can be moved. That's why you use DDR3 for the CPU, it matters how long it has to wait for many small chunks of data.

Here's an analogy. Two painters are each contracted to paint a picket fence.

Painter 1, GPU, is asked to paint the picket fence white. He needs a 5 gallon tub of paint to do it. His assistant, GDDR5, goes to the back of the garage to get the tub of paint then he proceeds with painting the fence. It wouldn't have made much difference if the 5 gallon tub was handy at the front of the garage. It would have only made the job go a little quicker. It was more important that GDDR5 was strong enough to carry the 5 gallon tub.

Painter 2, CPU, is asked to paint each picket a different colour. He needs many quarts of different colour paints to do it. His assistant, DDR3, goes to the garage and picks a quart of paint off the shelf at the front of the garage. Then cpu sends his assistant for the next quart of paint and this continues until the job is done. It wouldn't have made much difference if DDR3 was strong, it was more important that he was quicker.

Video games are far more GPU dependent than they are CPU dependent. This is why better bandwidth>better latency (and also why both consoles have a relatively weak CPU when compared against their GPUs). GPUs need lots and lots of bandwidth, as they have to constantly access VRAM to perform different operations, many of which take place at the same time and therefore require a lot of bandwidth so none of those operations starve each other of data when waiting to draw the next frame.

For those not technically knowledgeable, imagine it like this. GDDR5 is like having a 4 lane highway, with each car on that highway being a big van holding a bunch of passengers. DDR3 is more like a two lane highway with each car being a 2 seater sports car.

For tasks that require a lot of people to complete (like gaming), the 4 lane highway packed with vans full of people is going to give you better results. For smaller jobs that only require a couple people to complete, but need to be completed very quickly, the 2 lane road with sports cars on it will be the better approach. The faster sports car will get those 2 people where they are going faster.

Gaming happens to be one of those tasks that requires a lot of "people" working at the same time, so the 4 lane highway with vans on it is the better option. Xbox One will be shipping 2 people at a time to the job site, but they'll get there really fast due to their sports car. Thing is, once they realize they are doing a task that requires 4, 6, or 8 people, those 2 people will be stuck waiting on the other guys needed to complete the job to arrive as well. The PS4 will be unloading vans full of people each stop to complete the tasks, even though those vans will not be arriving as often as the sports cars will.

OS functions may run a bit slower on PS4 due to latency, or other general purpose computing tasks, but gaming most definitely reaps the benefits of the higher bandwidth approach. This machine was built from the ground up to be a gaming console, so that was the logical choice to make.

Edit: If you're going to disagree, at least leave a comment showing why. If you can't even detail why you don't agree, it shows you don't know enough about the topic to give your opinion in the first place.

Can I point out AMD are going to start selling APU's based on PS4's structure (with 4 instead of 8 core cpu's running at a slightly higher clock rate apparently)at the end of this year, and they have unified memory apparently. I'm assuming as they are based on PS4 this unified memory will be 4gb of GDDR5. So clearly somehow they've offset the issues as they plan to use some of this architecture for conventional pc's/budget gaming pc's.

@blackmagic, I actually like your analogy better, as it better clarifies RAM latency vs. mine, which makes the latency sound more like internet latency. Though in the end, it still equals how long the chip has to wait to get it's data.

The GPU is definitely the work horse in these new consoles. This is why we're seeing games rely on GPGPU functions more and more these days, offloading tasks that the CPU normally would do, like physics, onto the compute units of the GPU.

There are actually 2 things going for PS4 that make up for the added CPU latency of GDDR5. First, PS4's GPU has 6 more compute units, with 4 of them being specialized for performing GPGPU tasks and all of them having a direct bus to the CPU to aid it with calculations. Then you have the fact that PS4's CPU can be devoted entirely to gaming. It has it's own sub-processor to run it's OS, whereas the Xbox One has to use one of it's 8 CPU cores to run it's OS (I hear 2 of them are reserved for the system, I imagine kinect takes the other one?). PS4 has all 8 CPU cores to use for gaming, plus the additional 4 compute units on the GPU that can aid the CPU. As such, PS4's CPU will not be as much of a performance bottleneck as Xbox One's will, despite the higher RAM latency. The additional 2GBs of RAM available to games to use factors in there as well.

The latency complaints are getting out of hand. The fact of the matter is that GDDR5 is significantly better for powering games than normal DDR3. Latency has little to do with that.

The biggest complaint about latency you'll have would be OS features where simple things take a couple of seconds to load up, due to the high latency. But when it comes to games the bandwidth of the memory significantly outweighs any negatives latency could have on short calculation task.

BlackKnight...the thing is all this "latency hurts the CPU" talk is just talk...how many PC's have you seen running a unified pool of GDDR5?...yup, none...we can speculate on how latency might be an issue...but Cerny is one of the few that has actually experienced the performance...so his word has to be given more credibility then any arm chair engineers on N4G that think they are all knowing...

I just think Bungie don't want people to know that many people are developing for their game, raising higher standards/expectations etc for the game itself.

It's like when developers don't really want people to know their REAL budgets for their games, people use it as ammo as to why their game is good or bad etc and may have an effect on the critics/gamers perception, saying it's only good because of that budget, or should have been much better due to that etc etc

Cerny did his research, he didn't build the PS4 in a month, he has been working on it since 2008. If the latency were such a huge issue he would know better than anybody on this website because he is both: a game designer and hardware engineer.

Oh yea, when there were similar articles about xbox 360 and its move engines esram etc you all were saying " He's from microsoft, he's paid to say good things!!" but now when someone from sony talks good stuff about sony console hes 100% credible!

Latency over the internet between the cloud and latency difference between GDDR5 and DDR3 is much greater in the 1st instance. Therefore a hell of alot harder to get around not to mention the bandwidth which can be used to offset some of the latency issues i believe?

As much as people don't like Kutaragi for PS3 design he did pretty much CREATE the Playstation so a man has to give him the edge. Kutaragi had to convince the Sony execs to even consider a gaming system then designed the first ever 100m on his first try (people thought the NES's 60m was the ceiling before Sony changed the game). Then he followed up with the 160m selling PS2. PS3 got off to a rocky start and Ken didn't help with his complex expensive architecture but it will still most likely be the highest selling console of the gen by the time they stop selling.

Actually the original playstation was designed by Sony for nintendo as a CD add on for the super NES. But nintendo backed out the day they were supposed to go public about the nintendo playstation. Sony was going to scrap everything and kutaragi who was designing it convinced Sony to make it a stand alone CD based console. Then came the Sony Playstation era which still dominates.

No joke...the cpu section of the jaguar is the biggest bottle neck...but that will never be a factor because the ps4 isn't running a heavy OS, console development is never heavy on generic instruction sets...for calc intensive processing, the stronger gpu and high bandwidth ram (and plenty of it) will be very happy together...

people keep saying how crappy the ps4 cpu was originally made for budget laptops...remember though, the ps4 doesn't have to run windows AND a game...comparisons with PC hardware aren't exactly relevant...

"Latency in GDDR5 isn’t particularly higher than the latency in DDR3. On the GPU side… Of course, GPUs are designed to be extraordinarily latency tolerant so I can’t imagine that being much of a factor."

DDR5 doesn't even exist, DDR4 is only coming out starting from next year to the mainstream PC. And latency is not about being faster or slower but higher or lower.

GDDR5 has higher latency than DDR3 not slower. And even then, APU's CPU are weak and irrelevant to next-gen games anyway. It will be all about GPU and GPGPU implementation so the bandwidth will be a necessity. With the PS3 it was all about the Cell helping the RSX and with the PS4 the CPU will be there just to help the GPU and not to do anything else of importance.